1. Field of the Invention
The present invention relates to an image forming apparatus for transferring a toner image formed on an image bearing member onto a recording material and, more particularly, to an image forming apparatus including a rotatable brush that applies an electrical charge to residual toner remaining on the image bearing member.
2. Description of the Related Art
Recently, multi-function peripherals that include output terminal functions, such as copier, printer, and facsimile functions, in one body have been widely used. For these output terminals, electrophotographic image forming apparatuses have been widely used. For such electrophotographic image forming apparatuses, with an increase in the processing speed, the useful life thereof is expected to increase. In addition, for ecological reasons, waste generation needs to be reduced. That is, the number of disposables needs to be reduced and the life expectancy of disposables needs to be increased. Furthermore, the reliability of the disposables needs to be increased.
Accordingly, in order to sufficiently remove residual toner and increase the wear life of a photosensitive member, systems known as “cleaner-less systems” is employed. Cleaner-less systems include no cleaning units. After a transfer process is completed, a developing unit removes and recovers residual toner deposited on a photosensitive member using a “simultaneous development and recovery” technique. Thus, the toner is reused. In the simultaneous development and recovery technique, residual toner deposited on a photosensitive member is recovered in the subsequent development process. That is, in the subsequent development process, residual toner remaining in areas of the surface of the photosensitive member where an image should not be developed with toner (i.e., non-image portions) is recovered to a developing unit using a fog-removing voltage difference Vback, which is a difference between a direct current voltage applied to the developing unit and the surface voltage of the photosensitive member.
By using the simultaneous development and recovery technique, the need for a member that slides on a photosensitive member, such as a counter blade, can be eliminated. Thus, the wear life of the photosensitive member can be significantly increased. In addition, the cost can be reduced. In such a cleaner-less system, an auxiliary charging brush is disposed downstream of the primary transfer unit in order to control the polarity of the charge of the residual toner. The auxiliary charging brush is made from a conductive fiber. The auxiliary charging brush is disposed between the primary transfer unit and a charging unit. A voltage having a polarity the same as the charging polarity of the toner is applied to the auxiliary charging brush. Alternatively, a structure is widely used in which a brush including a conductive fiber to which a voltage having a polarity opposite to the charging polarity of the toner is applied is disposed upstream of a brush to which a voltage having a polarity the same as the charging polarity of the toner is applied and downstream of the primary developing unit in the rotational direction of the photosensitive member.
However, in the cleaner-less systems, a problem of an auxiliary charging brush filled with toner may occur. If the auxiliary charging brush is saturated with toner particles, an amount of electrical current flowing from the brush to a photosensitive drum decreases. For example, when paper jam occurs or an image requiring a large amount of toner is formed, a large amount of residual toner remains on the photosensitive member. In such a case, the brush is easily saturated with toner particles, and therefore, the amount of electrical current flowing into the photosensitive member significantly decreases. In particular, if the amount of electrical current flowing from the downstream brush decreases, it is difficult to control the polarity of the charge of the toner. Consequently, a large amount of the toner having charge of an opposite polarity may remain on the photosensitive drum. Furthermore, a charger roller may not be uniformly charged. Therefore, a defective image, such as a foggy image or a grainy foggy image, may be generated.
To prevent this problem, a fur brush that is not easily saturated with toner particles may be used. However, even when a fur brush is used, the fur brush may become saturated with toner due to environmental conditions or the deterioration thereof. In such a case, the toner particles need to be removed from the fur brush. To remove toner particles, a bias having a polarity opposite to that usually used for forming an image may be applied to the brush (refer to, for example, Japanese Patent Laid-Open No. 11-72995).
However, in the method for removing toner particles from the brush by applying a bias as described in Japanese Patent Laid-Open No. 11-72995, the toner particles may not be completely removed as the condition deteriorates.
This is because toner particles staying on the top end of the brush move towards the anchor end of the brush as the brush is used for a long term. The toner particles deposited on the top end of the brush can be removed by applying the bias. However, to remove the toner particles deposited on the anchor end of the brush, application of a high bias is needed. The withstand voltage of the brush is decreased as the brush is used for the long time. Thus, as the brush is used for the long time, it may be more difficult to remove the toner particles deposited in the brush.
Additionally, Japanese Patent Laid-Open No. 10-312098 describes a technique for preventing a brush from being saturated with paper dust. In this technique, paper dust is removed from the brush by supporting the brush in a rotatable manner and causing a drum to be rotated by the rotation of the brush. Alternatively, paper dust is removed from the brush by stopping the rotation of the brush during formation of an image and causing the brush to be rotated by the rotation of a drum when no images are formed. However, since the brush is rotated simply by the rotation of the drum, almost all toner particles in the brush cannot be removed. Thus, as the brush is used for a long term, the brush may be saturated with toner particles.
In addition, since this cleaner system does not include a cleaning unit for removing toner deposited on the surface of the drum, the deposited toner tends to cause a phenomenon known as filming in which the deposited toner becomes fused to the surface of the drum. If this phenomenon happens, a latent image is not completely formed on the drum. Thus, a defective image is produced. In particular, when the rotation speed of the fur brush serving as a charge supplying unit is high, filming may easily occur.